Various methods have been proposed to realize class D amplifiers (so-called digital amplifiers). Generally speaking, there are the method for realizing a digital system and the method for realizing an analog system. FIG. 7 is a diagram illustrating an example of the digital system. The design of the digital system is as follows: the input digital signal is usually oversampled, followed by ΔΣ modulation, and the obtained parallel multi-bit signal is converted by a digital/pulse-width modulator (digital PWM) to a 1-bit pulse-width modulation signal. In this way, a signal for driving the power section as the amplification section is obtained.
For the class D amplifier of the digital system, if the power section is an ideal switch, it is possible to realize the performance determined by the digital circuit up to the stage of the power section. However, the actual power section has several nonideal elements, so that in a real circuit, the performance of the class D amplifier is limited to the characteristics of the power section. Consequently, no matter how much the signal processing performance for a digital class D amplifier is improved, it is still impossible to realize performance better than the characteristics of the power section contained in the class D amplifier.
FIG. 8 is a diagram illustrating an example of an analog class D amplifier. In this analog system, an analog PWM circuit is used instead of the digital PWM, and the feedback from the output of the power section to the analog PWM circuit is used, so that the influence from nonideal elements of the power section is reduced, and, as a result, the performance of the class D amplifier can be significantly improved.
However, because the analog PWM circuit requires an analog input signal, a digital/analog converter is needed when handling the digital signals from CDs and DVDs, which have become the principal signal sources in recent years, as shown in the figure. Also, many digital/analog converters (DAC have noise outside the signal bandwidth, and, in order to remove the noise, as shown in the figure, it is necessary to insert a post low-pass filter (LPF) after the DAC. Because cutoff characteristics down to a very low band are needed for the low-pass filter, it is not easy to integrate the LPF in the IC circuit.    L. Risbo, T. Morch, Performance of an all-digital power amplification system, 104th Audio Engineering Society Convention, Amsterdam, pre print 4695.    1: Rudy J. Van de Plassche, “Dynamic element matching for high-accuracy monolithic D/A converters,” IEEE J. Solid-State Circuits, vol. SC-11, pp. 795-800, December 1976.    2: L. Richard Carley, “A noise-shaping coder topology for 15+ bi converters,” IEEE J. Solid-State Circuits, vol. SC-24, pp. 267-273, 1989.    3: Bosco H. Leung, “Architectures for multi-bit oversampled A/D converter employing dynamic element matching techniques,” IEEE ISCAS 1991, pp. 1657-1660.